Upgrade Handlebars to 1.0.rc.2

This upgrades Handlebars to the latest released version (1.0.rc.2).

The notable changes are the removal of the custom "yui-handlebars-utils.js" file, which is no longer needed because Handlebars has been updated to properly double-escape "&"s. This required changes to the import scripts, and ignoring of one test case for `Y.Handlebars.utils.isEmpty("")`, which now returns `true` (details in the commit messages.)